Zeolites have been widely studied for the conversion of methanol to hydrocarbons. Among them, ZSM-5 zeolite was the first zeolite studied as active catalyst for the conversion of oxygenates to hydrocarbons (D. Chang and A. J. Silvestri, Journal of Catalysis 47, 249-259 (1977)).
European patent application EP-A-0 448 000 relates to a process for the production of lower olefins from methanol by reacting a mixture containing methanol and/or dimethylether on crystalline aluminosilicates of the pentasil type having a Si/Al atomic ratio of at least 10. The only specifically disclosed catalyst is of the pentasil type and has a Si/Al atomic ratio of 103, a sodium content of about 340 ppm, a BET surface area of 342 m2/g and a pore volume of 0.33 cm3/g. This catalyst yields an olefin mixture of more than 6 wt.-% of ethylene, more than 40 wt.-% of propylene and less than 30 wt.-% of butenes from a methanol dimethylether mixture.
In the methanol to olefin conversion process deactivation of the catalyst by coke deposition has been a major drawback. The beneficial effect of decreasing the crystal size of the pentasil zeolites was shown EP-A-1 424 128 with higher selectivities to propylene and longer lifetime of the catalyst. Nonetheless the initial activity of the catalyst still is not fully recovered after regeneration when coke has been removed.
U.S. Pat. No. 3,911,041 discloses a catalyst comprising a crystalline aluminosilicate zeolite having a silica to alumina ratio of at least about 12 and a constraint index of about 1 to 12. The catalyst contains phosphorus incorporated with the crystal structure in an amount of about 3.5 to 4.8 wt.-%. The catalysts are used for the conversion of methanol and dimethylether to an olefin-containing reaction product. The methanol to olefin conversion is less than 2 wt.-%.
U.S. Pat. No. 5,573,990 discloses the conversion of methanol or dimethylether to light olefins by contacting methanol or dimethylether at a temperature of at least 400° C. with a zeolite ZSM-5 catalyst containing at least 0.7 wt.-% of phosphorus P and at least 0.97 wt.-% of rare earth element incorporated within the structure of the catalyst. The ZSM-5 based catalyst is prepared by: mixing a zeolite ZSM-5 catalyst with silica sol and an ammonium nitrate solution, kneading, moulding and calcining the mixture, ion exchanging with phosphoric acid under reduced pressure, drying and calcining the phosphorus modified zeolite, impregnating the phosphorus modified zeolite with a solution of rare earth elements under reduced pressure, drying and calcining the zeolite, and hydrothermally treating the obtained zeolite at 500-600° C. with water vapour.
U.S. Pat. No. 4,629,717 concerns a phosphorus modified alumina composite comprising a hydrogel having a molar ratio on an elemental basis of phosphorus to aluminum of 1:1 to 1:100 and a surface area of 140 to 450 m2/g. The composites are prepared by combining a phosphorus containing compound and an alumina hydrosol and gelling this mixture. This composite is used as a catalyst support in the hydrogenation of olefins.
U.S. Pat. Nos. 6,797,851 and 7,230,151 relate to a process of making olefins, particularly ethylene and propylene, from an oxygenate feed by use of two or more zeolite catalysts. The first catalyst may be a ZSM-5, the second catalyst contains a 10-ring molecular sieve, and is for example ZSM-22, ZSM-23, ZSM35 or ZSM-48. The amount of phosphorus, as measured on an elemental basis, is from 0.05 to 20 wt.-% based on the weight of the zeolite molecular sieve. The ZSM-5 catalyst may be unmodified, phosphorus modified or steam modified, however no specific catalysts or preparation methods thereof are disclosed.
U.S. Pat. No. 7,368,410 relates to a method for preparing zeolite catalysts comprising: treating a zeolite with a phosphorus compound to form a phosphorus-treated zeolite; heating the phosphorus-treated zeolite to a temperature of about 300° C. or higher; combining the phosphorus-treated zeolite with an inorganic oxide binder material to form a zeolite-binder mixture; and heating the zeolite-binder mixture at temperature of about 400° C. or higher to form a bound zeolite catalyst. The catalysts are used for the alkylation of aromatic compounds, especially for the methylation of toluene.
In EP-A-2 025 402 a phosphorus modified molecular sieve and its use in conversion of organics to olefins is disclosed. The molecular sieve may be prepared by: steaming a zeolite with a Si/Al ratio of below 30 at a temperature ranging from 550 to 680° C.; leaching with an aqueous phosphoric acid solution to remove a part of Al from the zeolite; separation of the zeolite from the liquid; optionally washing the zeolite; and calcining the zeolite.
The existing catalysts have the drawback of limited hydrothermal stability in the process for converting methanol into olefins. After extensive studies the present inventors were able to show that an advantageous stability can surprisingly be achieved by additionally impregnating a phosphorus containing zeolite catalyst with low amounts of phosphorus. These catalysts show an unexpected increase in the methanol to olefin (propylene) conversion process especially at low phosphorus content.